Rename the old TestPlatformVerifier to TestPlatformVerifierLegacy, and
add TODO about removing it once the synthetic root is widely deployed on
builders.
Updates #52108
Change-Id: I6cdba268e4738804c7f76ea18c354470b3e0318c
Reviewed-on: https://go-review.googlesource.com/c/go/+/505755
Run-TryBot: Roland Shoemaker <roland@golang.org>
Auto-Submit: Roland Shoemaker <roland@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Bryan Mills <bcmills@google.com>